Over the past two decades, energy storage technologies based on lithium-ion batteries have proven successful and found widespread use in many applications such as portable electronics and consumer devices. However, intercalation cathodes used in current lithium-ion batteries possess an inherent theoretical capacity limit of about 300 mAh g−1, which is a major factor limiting the specific energy of such batteries. These inherent theoretical constraints hinder the widespread use of lithium-ion batteries in many emerging applications such as vehicle electrification, thus impelling the pursuit of next-generation cathode materials with much higher specific capacities. Sulfur is a promising cathode material with a high theoretical capacity of about 1,673 mAh g−1 based on the electrochemical reaction: S8+16Li⇄8Li2S. There has been exciting progress in understanding and improving the electrochemical performance of sulfur cathodes. However, further progress is hindered by pairing with a lithium metal anode, which is prone to dendrite formation and other safety-related challenges.
Compared to sulfur, substantially fully-lithiated lithium sulfide (Li2S) (theoretical capacity of about 1,166 mAh g−1) represents a more attractive cathode material because it allows pairing with high-capacity lithium metal-free anodes (such as silicon or tin), hence obviating dendrite formation and safety concerns associated with metallic lithium. Moreover, the high melting point of Li2S (unlike that of sulfur) imparts greater ease of processing in the synthesis of carbon-based composite cathode materials. Furthermore, sulfur expands during lithiation, which can cause a surrounding material to crack and fracture. In contrast, Li2S contracts during delithiation, thereby mitigating against challenges resulting from volume expansion. Despite the promise, current efforts remain lacking in terms of achieving stable and high-performance Li2S cathodes. For example, the overall cycling performance and stability of Li2S cathodes remain generally poor, with typical cycle life of less than 100 cycles demonstrated in some reports.
It is against this background that a need arose to develop the multifunctional binders described herein.